/* The file system maintains a buffer cache to reduce the number of disk
 * accesses needed.  Whenever a read or write to the disk is done, a check is
 * first made to see if the block is in the cache.  This file manages the
 * cache.
 *
 * The entry points into this file are:
 *   get_block:	  request to fetch a block for reading or writing from cache
 *   put_block:	  return a block previously requested with get_block
 *   alloc_zone:  allocate a new zone (to increase the length of a file)
 *   free_zone:	  release a zone (when a file is removed)
 *   invalidate:  remove all the cache blocks on some device
 *
 * Private functions:
 *   rw_block:    read or write a block from the disk itself
 */

#include "fs.h"
#include <minix/com.h>
#include "buf.h"
#include "file.h"
#include "fproc.h"
#include "super.h"

FORWARD _PROTOTYPE( void rm_lru, (struct buf *bp) );
FORWARD _PROTOTYPE( int rw_block, (struct buf *, int) );

/*===========================================================================*
 *				get_block				     *
 *===========================================================================*/
PUBLIC struct buf *get_block(dev, block, only_search)
    register dev_t dev;		/* on which device is the block? */
    register block_t block;		/* which block is wanted? */
    int only_search;		/* if NO_READ, don't read, else act normal */
{
    /* Check to see if the requested block is in the block cache.  If so, return
     * a pointer to it.  If not, evict some other block and fetch it (unless
     * 'only_search' is 1).  All the blocks in the cache that are not in use
     * are linked together in a chain, with 'front' pointing to the least recently
     * used block and 'rear' to the most recently used block.  If 'only_search' is
     * 1, the block being requested will be overwritten in its entirety, so it is
     * only necessary to see if it is in the cache; if it is not, any free buffer
     * will do.  It is not necessary to actually read the block in from disk.
     * If 'only_search' is PREFETCH, the block need not be read from the disk,
     * and the device is not to be marked on the block, so callers can tell if
     * the block returned is valid.
     * In addition to the LRU chain, there is also a hash chain to link together
     * blocks whose block numbers end with the same bit strings, for fast lookup.
     */

    int b;
    register struct buf *bp, *prev_ptr;

    /* Search the hash chain for (dev, block). Do_read() can use
     * get_block(NO_DEV ...) to get an unnamed block to fill with zeros when
     * someone wants to read from a hole in a file, in which case this search
     * is skipped
     */
    if (dev != NO_DEV) {
	b = (int) block & HASH_MASK;
	bp = buf_hash[b];
	while (bp != NIL_BUF) {
	    if (bp->b_blocknr == block && bp->b_dev == dev) {
		/* Block needed has been found. */
		if (bp->b_count == 0) rm_lru(bp);
		bp->b_count++;	/* record that block is in use */

		return(bp);
	    } else {
		/* This block is not the one sought. */
		bp = bp->b_hash; /* move to next block on hash chain */
	    }
	}
    }

    /* Desired block is not on available chain.  Take oldest block ('front'). */
    if ((bp = front) == NIL_BUF) panic(__FILE__,"all buffers in use", NR_BUFS);
    rm_lru(bp);

    /* Remove the block that was just taken from its hash chain. */
    b = (int) bp->b_blocknr & HASH_MASK;
    prev_ptr = buf_hash[b];
    if (prev_ptr == bp) {
	buf_hash[b] = bp->b_hash;
    } else {
	/* The block just taken is not on the front of its hash chain. */
	while (prev_ptr->b_hash != NIL_BUF)
	    if (prev_ptr->b_hash == bp) {
		prev_ptr->b_hash = bp->b_hash;	/* found it */
		break;
	    } else {
		prev_ptr = prev_ptr->b_hash;	/* keep looking */
	    }
    }

    /* If the block taken is dirty, make it clean by writing it to the disk.
     * Avoid hysteresis by flushing all other dirty blocks for the same device.
     */
    if (bp->b_dev != NO_DEV) {
	if (bp->b_dirt == DIRTY) flushall(bp->b_dev);
#if ENABLE_CACHE2
	put_block2(bp);
#endif
    }

    /* Fill in block's parameters and add it to the hash chain where it goes. */
    bp->b_dev = dev;		/* fill in device number */
    bp->b_blocknr = block;	/* fill in block number */
    bp->b_count++;		/* record that block is being used */
    b = (int) bp->b_blocknr & HASH_MASK;
    bp->b_hash = buf_hash[b];
    buf_hash[b] = bp;		/* add to hash list */

    /* Go get the requested block unless searching or prefetching. */
    if (dev != NO_DEV) {
#if ENABLE_CACHE2
	if (get_block2(bp, only_search)) /* in 2nd level cache */;
	else
#endif
	    if (only_search == PREFETCH) bp->b_dev = NO_DEV;
	    else
		if (only_search == NORMAL) {
		    rw_block(bp, READING);
		}
    }
    return(bp);			/* return the newly acquired block */
}

/*===========================================================================*
 *				put_block				     *
 *===========================================================================*/
PUBLIC void put_block(bp, block_type)
    register struct buf *bp;	/* pointer to the buffer to be released */
    int block_type;			/* INODE_BLOCK, DIRECTORY_BLOCK, or whatever */
{
    /* Return a block to the list of available blocks.   Depending on 'block_type'
     * it may be put on the front or rear of the LRU chain.  Blocks that are
     * expected to be needed again shortly (e.g., partially full data blocks)
     * go on the rear; blocks that are unlikely to be needed again shortly
     * (e.g., full data blocks) go on the front.  Blocks whose loss can hurt
     * the integrity of the file system (e.g., inode blocks) are written to
     * disk immediately if they are dirty.
     */
    if (bp == NIL_BUF) return;	/* it is easier to check here than in caller */

    bp->b_count--;		/* there is one use fewer now */
    if (bp->b_count != 0) return;	/* block is still in use */

    bufs_in_use--;		/* one fewer block buffers in use */

    /* Put this block back on the LRU chain.  If the ONE_SHOT bit is set in
     * 'block_type', the block is not likely to be needed again shortly, so put
     * it on the front of the LRU chain where it will be the first one to be
     * taken when a free buffer is needed later.
     */
    if (bp->b_dev == DEV_RAM || block_type & ONE_SHOT) {
	/* Block probably won't be needed quickly. Put it on front of chain.
	 * It will be the next block to be evicted from the cache.
	 */
	bp->b_prev = NIL_BUF;
	bp->b_next = front;
	if (front == NIL_BUF)
	    rear = bp;	/* LRU chain was empty */
	else
	    front->b_prev = bp;
	front = bp;
    } else {
	/* Block probably will be needed quickly.  Put it on rear of chain.
	 * It will not be evicted from the cache for a long time.
	 */
	bp->b_prev = rear;
	bp->b_next = NIL_BUF;
	if (rear == NIL_BUF)
	    front = bp;
	else
	    rear->b_next = bp;
	rear = bp;
    }

    /* Some blocks are so important (e.g., inodes, indirect blocks) that they
     * should be written to the disk immediately to avoid messing up the file
     * system in the event of a crash.
     */
    if ((block_type & WRITE_IMMED) && bp->b_dirt==DIRTY && bp->b_dev != NO_DEV) {
	rw_block(bp, WRITING);
    }
}

/*===========================================================================*
 *				alloc_zone				     *
 *===========================================================================*/
PUBLIC zone_t alloc_zone(dev, z)
    dev_t dev;			/* device where zone wanted */
    zone_t z;			/* try to allocate new zone near this one */
{
    /* Allocate a new zone on the indicated device and return its number. */

    int major, minor;
    bit_t b, bit;
    struct super_block *sp;

    /* Note that the routine alloc_bit() returns 1 for the lowest possible
     * zone, which corresponds to sp->s_firstdatazone.  To convert a value
     * between the bit number, 'b', used by alloc_bit() and the zone number, 'z',
     * stored in the inode, use the formula:
     *     z = b + sp->s_firstdatazone - 1
     * Alloc_bit() never returns 0, since this is used for NO_BIT (failure).
     */
    sp = get_super(dev);

    /* If z is 0, skip initial part of the map known to be fully in use. */
    if (z == sp->s_firstdatazone) {
	bit = sp->s_zsearch;
    } else {
	bit = (bit_t) z - (sp->s_firstdatazone - 1);
    }
    b = alloc_bit(sp, ZMAP, bit);
    if (b == NO_BIT) {
	err_code = ENOSPC;
	major = (int) (sp->s_dev >> MAJOR) & BYTE;
	minor = (int) (sp->s_dev >> MINOR) & BYTE;
	printf("No space on %sdevice %d/%d\n",
		sp->s_dev == root_dev ? "root " : "", major, minor);
	return(NO_ZONE);
    }
    if (z == sp->s_firstdatazone) sp->s_zsearch = b;	/* for next time */
    return(sp->s_firstdatazone - 1 + (zone_t) b);
}

/*===========================================================================*
 *				free_zone				     *
 *===========================================================================*/
PUBLIC void free_zone(dev, numb)
    dev_t dev;				/* device where zone located */
    zone_t numb;				/* zone to be returned */
{
    /* Return a zone. */

    register struct super_block *sp;
    bit_t bit;

    /* Locate the appropriate super_block and return bit. */
    sp = get_super(dev);
    if (numb < sp->s_firstdatazone || numb >= sp->s_zones) return;
    bit = (bit_t) (numb - (sp->s_firstdatazone - 1));
    free_bit(sp, ZMAP, bit);
    if (bit < sp->s_zsearch) sp->s_zsearch = bit;
}

/*===========================================================================*
 *				rw_block				     *
 *===========================================================================*/
PRIVATE int rw_block(bp, rw_flag)
    register struct buf *bp;	/* buffer pointer */
    int rw_flag;			/* READING or WRITING */
{
    /* Read or write a disk block. This is the only routine in which actual disk
     * I/O is invoked. If an error occurs, a message is printed here, but the error
     * is not reported to the caller.  If the error occurred while purging a block
     * from the cache, it is not clear what the caller could do about it anyway.
     */

    int r, op;
    off_t pos;
    dev_t dev;
    int block_size;

    block_size = get_block_size(bp->b_dev);

    if ( (dev = bp->b_dev) != NO_DEV) {
	pos = (off_t) bp->b_blocknr * block_size;
	op = (rw_flag == READING ? DEV_READ : DEV_WRITE);
	r = dev_io(op, dev, FS_PROC_NR, bp->b_data, pos, block_size, 0);
	if (r != block_size) {
	    if (r >= 0) r = END_OF_FILE;
	    if (r != END_OF_FILE)
		printf("Unrecoverable disk error on device %d/%d, block %ld\n",
			(dev>>MAJOR)&BYTE, (dev>>MINOR)&BYTE, bp->b_blocknr);
	    bp->b_dev = NO_DEV;	/* invalidate block */

	    /* Report read errors to interested parties. */
	    if (rw_flag == READING) rdwt_err = r;
	}
    }

    bp->b_dirt = CLEAN;
}

/*===========================================================================*
 *				invalidate				     *
 *===========================================================================*/
PUBLIC void invalidate(device)
    dev_t device;			/* device whose blocks are to be purged */
{
    /* Remove all the blocks belonging to some device from the cache. */

    register struct buf *bp;

    for (bp = &buf[0]; bp < &buf[NR_BUFS]; bp++)
	if (bp->b_dev == device) bp->b_dev = NO_DEV;

#if ENABLE_CACHE2
    invalidate2(device);
#endif
}

/*===========================================================================*
 *				flushall				     *
 *===========================================================================*/
PUBLIC void flushall(dev)
    dev_t dev;			/* device to flush */
{
    /* Flush all dirty blocks for one device. */

    register struct buf *bp;
    static struct buf *dirty[NR_BUFS];	/* static so it isn't on stack */
    int ndirty;

    for (bp = &buf[0], ndirty = 0; bp < &buf[NR_BUFS]; bp++)
	if (bp->b_dirt == DIRTY && bp->b_dev == dev) dirty[ndirty++] = bp;
    rw_scattered(dev, dirty, ndirty, WRITING);
}

/*===========================================================================*
 *				rw_scattered				     *
 *===========================================================================*/
PUBLIC void rw_scattered(dev, bufq, bufqsize, rw_flag)
    dev_t dev;			/* major-minor device number */
    struct buf **bufq;		/* pointer to array of buffers */
    int bufqsize;			/* number of buffers */
    int rw_flag;			/* READING or WRITING */
{
    /* Read or write scattered data from a device. */

    register struct buf *bp;
    int gap;
    register int i;
    register iovec_t *iop;
    static iovec_t iovec[NR_IOREQS];  /* static so it isn't on stack */
    int j, r;
    int block_size;

    block_size = get_block_size(dev);

    /* (Shell) sort buffers on b_blocknr. */
    gap = 1;
    do
	gap = 3 * gap + 1;
    while (gap <= bufqsize);
    while (gap != 1) {
	gap /= 3;
	for (j = gap; j < bufqsize; j++) {
	    for (i = j - gap;
		    i >= 0 && bufq[i]->b_blocknr > bufq[i + gap]->b_blocknr;
		    i -= gap) {
		bp = bufq[i];
		bufq[i] = bufq[i + gap];
		bufq[i + gap] = bp;
	    }
	}
    }

    /* Set up I/O vector and do I/O.  The result of dev_io is OK if everything
     * went fine, otherwise the error code for the first failed transfer.
     */
    while (bufqsize > 0) {
	for (j = 0, iop = iovec; j < NR_IOREQS && j < bufqsize; j++, iop++) {
	    bp = bufq[j];
	    if (bp->b_blocknr != bufq[0]->b_blocknr + j) break;
	    iop->iov_addr = (vir_bytes) bp->b_data;
	    iop->iov_size = block_size;
	}
	r = dev_io(rw_flag == WRITING ? DEV_SCATTER : DEV_GATHER,
		dev, FS_PROC_NR, iovec,
		(off_t) bufq[0]->b_blocknr * block_size, j, 0);

	/* Harvest the results.  Dev_io reports the first error it may have
	 * encountered, but we only care if it's the first block that failed.
	 */
	for (i = 0, iop = iovec; i < j; i++, iop++) {
	    bp = bufq[i];
	    if (iop->iov_size != 0) {
		/* Transfer failed. An error? Do we care? */
		if (r != OK && i == 0) {
		    printf(
			    "fs: I/O error on device %d/%d, block %lu\n",
			    (dev>>MAJOR)&BYTE, (dev>>MINOR)&BYTE,
			    bp->b_blocknr);
		    bp->b_dev = NO_DEV;	/* invalidate block */
		}
		break;
	    }
	    if (rw_flag == READING) {
		bp->b_dev = dev;	/* validate block */
		put_block(bp, PARTIAL_DATA_BLOCK);
	    } else {
		bp->b_dirt = CLEAN;
	    }
	}
	bufq += i;
	bufqsize -= i;
	if (rw_flag == READING) {
	    /* Don't bother reading more than the device is willing to
	     * give at this time.  Don't forget to release those extras.
	     */
	    while (bufqsize > 0) {
		put_block(*bufq++, PARTIAL_DATA_BLOCK);
		bufqsize--;
	    }
	}
	if (rw_flag == WRITING && i == 0) {
	    /* We're not making progress, this means we might keep
	     * looping. Buffers remain dirty if un-written. Buffers are
	     * lost if invalidate()d or LRU-removed while dirty. This
	     * is better than keeping unwritable blocks around forever..
	     */
	    break;
	}
    }
}

/*===========================================================================*
 *				rm_lru					     *
 *===========================================================================*/
PRIVATE void rm_lru(bp)
    struct buf *bp;
{
    /* Remove a block from its LRU chain. */
    struct buf *next_ptr, *prev_ptr;

    bufs_in_use++;
    next_ptr = bp->b_next;	/* successor on LRU chain */
    prev_ptr = bp->b_prev;	/* predecessor on LRU chain */
    if (prev_ptr != NIL_BUF)
	prev_ptr->b_next = next_ptr;
    else
	front = next_ptr;	/* this block was at front of chain */

    if (next_ptr != NIL_BUF)
	next_ptr->b_prev = prev_ptr;
    else
	rear = prev_ptr;	/* this block was at rear of chain */
}
